the possibilities of substitution for scarce materials and energy sources. On the other hand, institutional changes, such as the adoption of countercyclical fiscal and monetary mechanisms, have removed much of the force of the pronatalist sentiment that was based on stagnationist arguments. Moreover, while the low fertility characterizing the developed countries is consistent with vital rates and age distributions that vary appreciably, the magnitude of the feasible changes (taking each country as a unit) is nevertheless fairly limited when looked at from the standpoint of short-term economic effects. Within the framework of these considerations however, there appears to exist on balance a tentative concensus that population growth at a moderately low rate has some advantages over a zero rate of growth: as a minimum it lessens the need for economic arrangements to compensate for the disadvantages of a stationary age structure. A Lower Optimal Population. This perspective on growth and on the related age distribution is, however, substantially altered if the implied long-term consequences on population size and density are taken into account. On purely economic grounds the prospects of an increase in size and density seem to appear in a mildly favorable light only in a relatively few advanced countries, and there only if the possibilities of relying on international trade to reap the advantages of specialization and large-scale production are ignored. More significantly, a number of interrelated and overlapping considerations suggest that the very successes of economic growth tend to impose increasing penalties on further demographic expansion.* On the somewhat loose notion of classical population theory, it may be suggested that, on balance, continued economic growth and continued current trends in technological and organizational changes (including trends toward a more integrated world economic system) will tend to lower the population that is optimal for individual countries. The production of high levels of income per head involves a multiplication of physical artifacts of a staggering variety that create unprecedented demands on space, energy, and raw materials per head. The process of consumption itself has similar effects. Indeed, resource problems in the richest countries have tended to reappear, ironically, in the reverse form of the classical resource problems, i.e., as problems of disposing of a wide range of waste by-products. The pollution of the environment that results, and the costs of coping with such pollution, tend to increase more than proportionately and often discontinuously as the volume of discharge increases. The point is not that population growth is alone or even primarily responsible for the appearance of such problems but merely that the size of a problem for any given income level tends to be at least proportionate to population size. Once a *For relevant discussions and some pertinent obiter dicta see (17-22).